Capacitor test cell for oxidation testing of oil



y 1964 F. c. DOBLE ETAL CAPACITOR TEST CELL FOR OXIDATION TESTING OF OILOriginal Filed Sept. 25, 1959 United States Patent 3,134,936 CAPACITORTEST CELL FOR OXIDATlON TESTING OF OIL Frank C. Dobie, Belmont, andFrederick S. Oliver,

Reading, Mass., assignors to Doble Engineering Company, Belmont, Mass, acorporation of Massachusetts Original application Sept. 25, 1959, Ser.No. 842,497, new Patent No. 3,037,164, dated May 29, 1962. Divided andthis application Sept. 27, 1961, Ser. No. 146,680

2 Claims. (Cl. 317246) This invention relates to accelerated oxidationtesting of insulating oil and, more particularly, it is concerned withmeasurements indicative of an A.-C. loss characteristic of the oilduring the course of an accelerated-oxidation test. This application isa division of our application Serial No. 842,497, filed September 25,1959, now Patent No. 3,037,164.

A disadvantage of oil as an insulator is that it deteriorates in use sothat after a time it must be replaced in order to minimize the risk ofan electrical failure. This is especially important in power systemswhere insu-lating oils are used in such devices as transformers,capacitors, cables, and switch gear, and where a single failure mayresult in a substantial interruption of service. The problem is how todetermine the point at which an insulating oil is no longer safe to use.Heret-ofore changes in the A.-C. loss characteristics of an oil havebeen found to provide a good indication of its electrical state providedthat A.-C. loss characteristics measurements are made often enough todefine the changes accurately.

According to the present invention, it is proposed to provide novelmethods and apparatus capable of performing accelerated life tests oninsulating oil samples, and thereby determine in advance an oils A.-C.loss characteristics over its useful life. Thus, these tests providevaluable information as to the relative merits of different kinds ofoils as useful insulators in advance and without risking damage toexpensive power equipment, as well as permitting the selection of an oilhaving preferred characteristics for a specified service. These novelresults are accomplished in the present invention by accelerating thedeterioration of insulating oil samples by continuously passing airthrough them in the presence of a metallic catalyst such as copper oriron or both, and then periodically measuring the A.-C. losscharacteristics of the oil at the same time and in the same cell whereinthe oil is undergoing continuous oxidation. This novel technique permitsa high degree of control as contrasted with conventional techniqueswhere samples must be removed for test, and thus only occasional testsare made which are subject to the errors attendant on any samplingmethod.

In order most effectively to carry out the methods of the invention,-there is also provided novel apparatus including a novel test cellespecially adapted for simultaneous electrical measurement andoxidation, to establish its A.-C. loss characteristics throughout itsuseful life, usually until a precipitate or sludge first appears.

It is an especially important object of the present invention to providehighly precise apparatus of this character.

The novel features of the invention together with fur ther objects andadvantages thereof will become apparent from the following detaileddescription of preferred embodiments thereof and from the drawings towhich it refers.

In the drawings:

FIG. 1 is a graph showing A.-C. loss characteristicst-ime curves ofvarious oils while undergoing continuous oxidation according to thepresent invention; and

FIG. 2 is a sectional view of a novel continuous oxidation test cellaccording to the present invention.

Referring to the drawings, in FIG. 1 is shown a number of typical A.-C.loss characteristics-time curves of oils measured by the methods of thepresent invention utilizing, say, a test cell such as is shown in FIG.2. Such test cell, generally designated 8, includes a central downwardlyextending air tube 10, a surrounding coil 11, or coils, usually ofcopper or iron or both, an inner electrode '12, outer electrode 13 andmiddle electrode 14, these elements being immersed in the oil to betested contained within a suitable surrounding jar 9.

-In accordance with the methods of the invention for evaluating oil interms of an A.-C. loss characteristics time tests, the body of oil to bemeasured is maintained in jar 9 with its level above the tops of theelectrodes 12, 13 and "14 as is shown in (FIG. 2, and may be heated ifdesired to achieve a uniform temperature, C. being common. A continuousstream of an oxygen-containing gas, usually air, is continuously passedfrom a suitable source, not shown, through central tube 10 and emergesfrom said tube through an accurately dimensioned jet adjacent the bottomof the container at the lower end of the concentric electrodes in smallbubbles or other finely divided form for free flow vertically upwardlythrough the body of the oil in the catalyst-containing volume betweenthe tube 10 and inner electrode 12 generally along the central verticalcommon axis of the concentric electrodes. The rate of flow is keptuniform by maintaining the same pressure on each accurately dimensionedjet and, therefore, each and every tube is exposed to the same rate ofair flow. The upward movement of the bubbles also causes a flow of oilas is shown by the arrows in FIG. 2 around the top of the innerelectrode and downwardly between the intermediate electrode and both theinner and outer electrodes. Thus, the electrical measuring element,basically the middle and outer electrodes, since the inner electrode iselectrically guard-grounded, is remote from the upwardly flowingbubbles, any electrical measurements taken between the intermediateelectrode and the outer electrode are not affected by the continuousflow of gas upwardly through the cell, even though they are continuouslyWashed by oil flowing from the oxidation region of the cell to theremote electrical measuring region thereof and downwardly past themiddle and outer electrodes. Under these conditions, it is possible tomeasure an electrical loss characteristic of the oil, such as itsdissipation factor, simultaneously with continuous oxidation of the oilin the same cell.

More specifically as to cell 8, its three electrodes 12, 13 and 14 areseen to comprise hollow cylinders joined by spacer members 121, 122 in acoaxial array. Spacer members 121, 122 are made of a suitable insulatingmaterial such as styrene plastic whereas the electrodes themselves areof inert metal such as stainless steel or plated active metals such asbrass so that they will be inert in the presence of oil although one ormore of the electrodes may be copper and used to replace the separatecatalyst coil. Also, it should be noted that the outer and middleelectrodes 13 and 14 are each directly supported from the innerelectrode 12 by their spacers 121 and 122 respectively. This isimportant in order to measure only the characteristics of the oil. Theinner electrode :12 is supported on tube 10 by a metallic spacer 123 sothat it is electrically connected to said tube for guard-groundingtherethrough. The oil is held in a jar 9 of glass wherein the abovedescribed electrode assembly is suspended by means of a metallic topplate 124 and is submerged beneath the surface of the oil. In order toaccelerate the process of deterioration that the oil normally undergoesin service, there is provided a catalyst 11 within the inner electrodewhich may be in the form of a coil 11 of copper or iron wire or both,and dry air is passed through it. The catalyst may conveniently taketheiorm of a bifilar helix 11 of iron-and copper which is retained onair tube 10. The tube is supported coaxially within the electrode 12 byits spacer 123 and hasitsoultlet end slightly above the lower end of jar9. As a consequence,v air bubbles pass up through the grounded innerelectrode 12' wherezthe turbulence created thereby has substantiallynoeffect on the electrical measurement of ,A.-C. loss characteristicsconducted. in a region remote therefrom inthe dielectric space betweenelectrodes '13 and 14. It has also been found desirable to elevate the.temperature of the cell and others like it. This may be accomplished bythe provision of a tank ofhot liquid (not shown) in which the containersare immersed;

Thoseskilled in thear-t will appreciate that the apparatusdescribedindetail herein byway of example can be modified in various ways-withoutdeparting from the spirit and scope, of the invention.

Whattis claimed is:

'1. A cell ,for measuring an electrical loss characteristic ofwanelectrical'insulating oilwhile undergoing continuous oxidationcomprising a container for maintainingtherein a bodyof oil to bemeasured, an electrode system immersed in said body of oil, said system.consisting of at least 'two vertically arranged radiallyspacedelectrodes ingenerally coaxialrelationship to one another andhavin-ga central vertical axis, gas supply means having a gas outletopening located at aposition substantially below the .top ofv at leastthe inner of saidelectrodes and on the central vertical axis of saidelectrode system supplying a stream of oxygen-containing gas to saidoil, and vertically spaced upper and lower passageway meanscommunicating with a central region within the inner of said electrodesand surrounding said central axis and a region between said spacedconcentric electrodes, said cell providing for free flow of said gasupwardly in a stream generally vertically along said central axis, saidelectrodes surrounding said central axis and said stream of gas, saidstream of gas being maintained ent-irelywithin References Cited in-thefile of thispatent UNITED STATES PATENTS 2,599,583 Robinson et al June10, 1952 2,752,543 Smith June 26, 1956 2,800,628 Stinson July 23, 1957FOREIGN PATENTS 941,082 Germany Apr. 5, 1956

1. A CELL FOR MEASURING AN ELECTRICAL LOSS CHARACTERISTIC OF ANELECTRICAL INSULATING OIL WHILE UNDERGOING CONTINUOUS OXIDATIONCOMPRISING A CONTAINER FOR MAINTAINING THEREIN A BODY OF OIL TO BEMEASURED, AN ELECTRODE SYSTEM IMMERSED IN SAID BODY OF OIL, SAID SYSTEMCONSISTING OF AT LEAST TWO VERTICALLY ARRANGED RADIALLY SPACEDELECTRODES IN GENERALLY COAXIAL RELATIONSHIP TO ONE ANOTHER AND HAVING ACENTRAL VERTICAL AXIS, GAS SUPPLY MEANS HAVING A GAS OUTLET OPENINGLOCATED AT A POSITION SUBSTANTIALLY BELOW THE TOP OF AT LEAST THE INNEROF SAID ELECTRODES AND ON THE CENTRAL VERTICAL AXIS OF SAID ELECTRODESYSTEM SUPPLYING A STREAM OF OXYGEN-CONTAINING GAS TO SAID OIL, ANDVERTICALLY SPACED UPPER AND LOWER PASSAGEWAY MEANS COMMUNICATING WITH ACENTRAL REGION WITHIN THE INNER OF SAID ELECTRODES AND SURROUNDING SAIDCENTRAL AXIS AND A REGION BETWEEN SAID SPACED CONCENTRIC ELECTRODES,SAID CELL PROVIDING FOR FREE FLOW OF SAID GAS UPWARDLY IN A STREAMGENERALLY VERTICALLY ALONG SAID CENTRAL AXIS, SAID ELECTRODESSURROUNDING SAID CENTRAL AXIS AND SAID STREAM OF GAS, SAID STREAM OF GASBEING MAINTAINED ENTIRELY WITHIN THE INNERMOST OF SAID ELECTRODES TOCAUSE UPWARD FLOW OF OIL IN THE REGION WITHIN SAID INNERMOST ELECTRODEAND DOWNWARD FLOW OF OIL IN THE REGION OUTWARDLY OF SAID INNERMOSTELECTRODE, AND RADIAL FLOW THROUGH SAID PASSAGEWAY MEANS TO RECIRCULATESAID OIL.